1. Field of the Invention
The invention relates to the field of optical disk recorders and, more particularly, to means for verifying the proper writing of data on the optical disk.
2. Brief Description of the Prior Art
Prior read after write verify systems have used comparators to compare the read signal immediately after a hole has been written with a predetermined reference value. This reference level is a so-called predetermined threshold value. See, e.g., Miyauchi et al, U.S. Pat. No. 4,308,612; issued Dec. 29, 1981; filed Dec. 20, 1979; at col. 4, lines 28-33 and lines 48-52. For a practical optical recording systems, this method may not be entirely satisfactory due to the fact that the hole associated signal power may spread far beyond the location of the hole. It may be present at significant levels several positions away, at a point at which the sampling for a subsequent hole may be taking place. By positions we mean symbol positions in a fixed-block symbol. In writing holes on an optical disk without using a fixed-block symbol, positions may be considered to be the minimum spacing between holes. When the hole associated power from one hole is present to a significant extent at a subsequent sampling point for a subsequent hole, comparing hole associated power against an absolute predetermined threshold may inadvertently indicate the correct writing of the subsequent hole when, in fact, no such hole was written.
In addition, setting such a threshold may not be practicable in light of the many variations in optical system parameters that must be taken into account, including variations in disk reflectivity both from disk to disk and across a given disk, variations in laser power, optical efficiency, sense diode coupling error and circuit tolerances. Using a threshold in this environment will degrade signal to noise ratio margins.